Ruthenium recovery process by solvent extraction

ABSTRACT

Process for the recovery of the ruthenium present in an aqueous nitric solution, wherein the ruthenium is extracted in an organic solvent by contacting the nitric solution with an organic phase comprising an organophosphorous compound having at least one electron donor sulphur atom in the presence of a compound able to displace NO +  ions of the ruthenium complexes present in the nitric solution. Application to the recovery of ruthenium contained in a nitric solution obtained by dissolving irradiated fuel elements.

BACKGROUND OF THE INVENTION

The invention relates to a process for the recovery of ruthenium presentin an aqueous nitric solution and relates to a process for recoveringthe radioactive ruthenium present in a nitric solution obtained bydissolving irradiated nuclear fuel elements.

Conventional processes for the reprocessing of irradiated nuclear fuelelements generally involve a first stage of dissolving the nuclear fuelby means of nitric acid, which leads to the obtainment of a nitricsolution containing not only uranium and plutonium but also certainfission products, particularly ruthenium contained in by no meansnegligible quantities in irradiated fuels. Thus, one ton of irradiateduranium in a light water nuclear reactor contains approximately 2300 gof ruthenium.

Thus, considerable interest in attached to the recovery of ruthenium notonly in order to ensure a satisfactory purification of the uranium andplutonium, but also due to the catalytic and photochemical metallurgicalproperties of ruthenium, which make it an element sought in numerousapplications. However, in irradiated fuel processing processes therecovery of ruthenium causes certain problems, because during the firsturanium and plutonium extraction cycle in an organic solvent, such astributyl phosphate, the ruthenium fraction which is also extracted inthis solvent is only re-extractable with difficulty and the solventsused consequently have a high residual activity.

It is also desirable to quantitatively recover the ruthenium beforesubjecting the nitric dissolving solution to a first uranium andplutonium extraction cycle.

The presently known processes for recovering ruthenium from nitricsolutions comprise transforming the ruthenium complexes present in thesesolutions into volatile ruthenium tetraoxide RuO₄. This rutheniumvolatilization is generally carried out by means of ozone or otheroxidizing agents such as anions, e.g. permanganates, dichromates,periodates, chromates, metal cations such as silver, sodium (IV), cobaltor lead dioxide.

The process for the recovery of the ruthenium present in a nitricsolution is also known which consists of adding nitride ions to thesolution and passing ozones through this solution in order to form andvolatilize the ruthenium tetroxide.

However, these processes have certain disadvantages.

Thus, it is necessary to heat the solution to a temperature of about 95°C. Moreover, said processes require the adding of catalysts to thenitric solution and only permit a quantitative recovery of ruthenium bybeing carried out over a relatively long period lasting approximately 12hours.

A process for the extraction of ruthenium from aqueous solutions bymeans of organic solvents in the presence of nitride ions is also known.

It has also been envisaged to extract ruthenium from hydrochloricsolutions by means of thiophosphoric acids.

However, these processes do not permit a quantitative recovery of theruthenium from nitric solutions.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a process for the recovery of rutheniumwhich obviates the above disadvantages.

The process according to the invention for the recovery of rutheniumpresent in an aqueous nitric solution is characterized in that theruthenium is extracted in an organic solvent by bringing the said nitricsolution into contact with an organic phase comprising anorganophosphorous compound having at least one electron donor sulphuratom in the presence of a compound able to displace the NO⁺ ions of theruthenium complexes present in the nitric solution.

According to the invention, the organophosphorous compound isadvantageously a dialkyl-dithiophosphoric acid such asdi-(2-ethyl-hexyl)-dithiophosphoric acid.

The above process has the advantage of leading to a quantitativerecovery of the ruthenium present in a nitric solution obtained forexample by dissolving irradiated nuclear fuel elements. Thus, in thiscase, the uranium VI and the plutonium are not in practice extracted inthe organic solvent, so that it is possible to recover the rutheniumwith satisfactory yields.

According to the invention, the compound which is able to displace theNO⁺ ions of the ruthenium complexes present in the nitric solution isadvantageously sulphamic acid or hydrazine.

When sulphamic acid is used, the latter is preferably added to theaqueous phase in a quantity such that the sulphamic acid concentrationof the aqueous phase is between 0.05 and 1 m/l.

Advantageously, extraction takes place at a temperature between 20° and80° C., preferably at 70° C.

When the ruthenium is extracted by means ofdi-(2-ethyl-hexyl)-dithiophosphoric acid in the presence of sulphamicacid a ruthenium complex is formed in the organic phase which inultraviolet spectrometry has three absorption bands at 515, 415 and 360nanometers doubtless corresponding to the extraction of a complexaccording to the following formula: ##STR1## in which R represents the2-ethyl-hexyl radical.

It is considered that the extraction mechanism corresponds to thefollowing reaction diagram: ##STR2##

According to a variant of the process of the invention the rutheniumextraction is carried out by means of an organic phase comprising anorganophosphorous compound containing at least one electron donorsulphur atom and one quaternary ammonium salt, such astricapryl-methyl-ammonium chloride.

According to this variant of the process of the invention, the rutheniumextraction coefficient is improved, doubtless due to the fact that theruthenium present in nitric solution can exist with different valenciesand because the presence of the quaternary ammonium salt thus makes itpossible to carry out the extraction of anionic complexes correspondingto ruthenium valence 2.

In this variant of the process of the invention the extraction is alsoperformed in the presence of a compound able to displace the NO⁺ ions ofthe ruthenium complexes, advantageously at a temperature between 20° and80° C., preferably at 70° C.

In this case, the ruthenium complex extracted in the organic phase alsohas in ultraviolet spectrometry three absorption bands at 550, 465 and370 nanometers. It is assumed that in this case the extraction mechanismcorresponds to the following reaction diagram: ##STR3##

For performing the process of the invention, the organic solventadvantageously comprises an inert diluent, such as dodecane.

It is pointed out that the process according to the invention can beperformed in any conventional extraction apparatus and in particular inmixer-settlers, pulsed columns, centrifugal extractors, etc.

DESCRIPTIONS OF DRAWINGS AND EXAMPLES

The invention is described in greater detail hereinafter relative tonon-limitative examples and with reference to the attached drawings,wherein show:

FIGS. 1, 2 and 3 graphs representing the variations in the distributionor partition coefficient D of the ruthenium as a function of the time inminutes in the case of an extraction performed by means of an organicsolvent constituted by di-(2-ethyl-hexyl)-dithiophosphoric acid dilutedin dodecane, FIGS. 1, 2 and 3 illustrating respectively the effect ofthe temperature, the addition of sulphamic acid and the addition ofhydrazine on the partition coefficient D.

FIG. 4 a graph showing variations of the partition coefficient D ofruthenium as a function of the nitric acid concentration of the startingsolution during an extraction process performed by means ofdi-(2-ethyl-hexyl)-dithiophosphoric acid.

FIG. 5 a diagram showing variations in the partition coefficient D ofruthenium as a function of time and illustrating the effect of thepresence of uranyl nitrate in the starting solution.

FIG. 6 a diagram showing variations in the partition coefficient D ofthe ruthenium as a function of time in the case of an extraction processperformed by means of an organic solvent containingdi-(2-ethyl-hexyl)-dithiophosphoric acid and tricapryl-methyl-ammoniumchloride.

EXAMPLE 1

This example relates to the recovery of ruthenium from a 3 N nitric acidsolution containing 2.6 ·10⁻³ M/liter of radioactive ruthenium in theform of mixed nitrato and nitrosoruthenium complexes at valencies II,III and IV.

In this example, an organic solvent constituted bydi-(2-ethyl-hexyl)-dithiophosphoric acid (DEHDTP) diluted in dodecane isused for extracting the ruthenium, the DEHDTP concentration of theorganic phase being 0.5 M/liter.

Extraction is carried out in an apparatus which is thermostaticallycontrolled by water circulation by bringing into contact within theapparatus 20 cm³ of aqueous phase and 20 cm³ of organic phase and bystirring the two phases in the presence of at least one rotary barmagnet.

During extraction, the phases present are sampled accompanied bystirring and after separating them by centrifuging each of them isanalysed by gamma spectrometry in order to determine their respectiveruthenium concentrations, which makes it possible to calculate thedistribution or partition coefficient D of the ruthenium which is equalto the ratio of the ruthenium concentration of the organic phase to theruthenium concentration of the aqueous phase.

In a first series of experiments, this extraction is carried out byusing a solvent after adding to the aqueous phase 0.25 M/l of sulphamicacid and extraction is performed at temperatures of 20° C., 50° C. and70° C., whilst determining the ruthenium partition coefficient as afunction of time for each temperature.

The results obtained are firstly given in FIG. 1, which shows thevariations in the partition coefficient D of ruthenium as a function oftime in minutes for extractions carried out respectively at temperaturesof 70° C. (curve I), 50° C. (curve II) and 20° C. (curve III).

It can be seen that the partition coefficient increases withtemperature, the best results being obtained when extraction isperformed at 70° C.

In a second series of experiments, ruthenium extraction is carried outeither at 70° C. or at 50° C. from aqueous phases having differentsulphamic acid concentrations, whilst once again determining in eachcase the partition coefficient D of the ruthenium as a function of time.

The results obtained are given in FIG. 2, which illustrates thevariations in the partition coefficient D as a function of time forextraction processes carried out at 70° C. with sulphamic acidconcentrations of 0.5 M/l (curve I) and 0.01 M/l (curve III) and forextraction processes carried out at 50° C. with sulphamic acidconcentrations of 0.4 M/l (curve II) and in the absence of sulphamicacid (curve IV).

It can be seen that the partition coefficient D is low in the absence ofsulphamic acid (curve IV) and that the addition of sulphamic acid makesit possible to improve this partition coefficient.

Moreover, it has been found that on the basis of a sulphamic acidconcentration of 0.4 M/l, the ruthenium extraction kineticssubstantially do not change.

In a third series of experiments, ruthenium extraction is carried out byadding hydrazine to the aqueous phase, extraction being performed at 70°C.

The results obtained are given in FIG. 3, which shows the variations inthe partition coefficient D of ruthenium as a function of time forextractions carried out on the basis of aqueous phases havingrespectively hydrazine concentrations of 0.1 M/l (curve I) and 0.01 M/l(curve II).

It can be seen that hydrazine also leads to an improvement in thepartition coefficient D, but that it is less effective than sulphamicacid.

EXAMPLE 2

This example relates to ruthenium recovery from a nitric solution with aruthenium concentration of 2.6·10⁻³ M/liter and a sulphamic acidconcentration of 0.5 M/liter.

In this example, extraction is carried out at a temperature of 50° C. bycontacting 20 cm³ of nitric solution and 20 cm³ of organic solventconstituted by di-(2-ethyl-hexyl)-dithiophosphoric acid diluted indodecane, the DEHDTP concentration of the organic phase being 0.5 M.After extracting for 6 hours accompanied by stirring, the two phases areseparated by centrifuging and the partition coefficient D of theruthenium is determined as in example 1.

The results obtained on varying the nitric acid concentration of thestarting solution are given in FIG. 4, which shows the variations in thepartition coefficient D of the ruthenium as a function of the nitricacid concentration of the aqueous phase.

These results show that the partition coefficient D varies little whenthe nitric acid concentration is between 0.3 and 3 N, but decreases veryrapidly when the nitric acid concentration exceeds 5 N.

EXAMPLE 3

In this example, ruthenium is covered from a 3 N solution containing2.6·10⁻³ M/liter of ruthenium, 0.5 M/l of uranyl nitrate and 0.25 M/l ofsulphamic acid.

Extraction is performed at 50° C. using the same organic solvent as inExample 1. The partition coefficient D of the ruthenium is alsodetermined as a function of time as in Example 1.

The results obtained are given in FIG. 5 in which curve I represents thevariations in the partition coefficient D of the ruthenium as a functionof time, when the aqueous phase contains uranyl nitrate, curve IIcorresponding to the same experiment performed in the absence of uranylnitrate.

It can be seen that the partition coefficients D are comparable and thaturanium has no effect on the extraction kinetics. Moreover, it should benoted that the uranium is not extracted in the organic solvent, thepartition coefficient D of the uranium being 3·10⁻² after 300 minutes ofextraction.

EXAMPLE 4

This example relates to the recovery of ruthenium from a 3 N nitricsolution containing 2.6·10⁻³ M/l of ruthenium.

In this example, the organic solvent is constituted by a mixture ofdi-(2-ethyl-hexyl)-dithiophosphoric acid (DEHDTP) andtricapryl-methyl-ammonium chloride diluted in dodecane, the DEHDTPconcentration of the organic phase being 0.5 M and thetricapryl-methyl-ammonium chloride concentration of the organic phasebeing 0.1 M.

In this example, extraction is carried out at a temperature of 20° or50° C., after adding or not adding sulphamic acid and optionally uranylnitrate to the aqueous phase. In each case, the partition coefficient Dof the ruthenium is determined as a function of time as in Example 1.

The results obtained are given in FIG. 6, which represents thevariations of the partition coefficient D of the ruthenium as a functionof time, for extraction processes performed under the followingconditions:

Curve I; Aqueous phase containing 0.25 M/l of sulphamic acid, extractionis performed at 50° C.

Curve II (for information): Aqueous phase containing 0.25 M/l ofsulphamic acid, but extraction performed by means of DEHDTP only at 50°C.

Curve III: Aqueous phase containing 0.25 M/l of sulphamic acid,extraction performed at 20° C.

Curve IV: Aqeuous phase containing 0.25 M/l of sulphamic acid and 0.5M/l of uranyl nitrate, extraction performed at 50° C.

Curve V: Aqueous phase not containing sulphamic acid, extractionperformed at 50° C.

The addition of a single quaternary ammonium salt, such astricapryl-methyl-ammonium chloride to the DEHDTP makes it possible toimprove the ruthenium partition coefficient, except when the aqueousphase also contains uranium, so that it can be assumed that thequaternary ammonium salt reacts with the uranyl nitrate and no longerfulfills its extractant function with respect to ruthenium II.

The effect of temperature is the same as when using an organic solventonly containing DEHDTP diluted in dodecane, the partition coefficient Dalso increasing with the temperature.

The addition of sulphamic acid to the aqueous phase makes it possible toimprove the partition coefficient.

What is claimed is:
 1. A process for the recovery of the rutheniumpresent in an aqueous nitric solution, wherein the ruthenium isextracted in an organic solvent by contacting the nitric solution withan organic phase comprising a dialkyldithiophosphoric acid in thepresence of a compound able to displace NO⁺ ions of the rutheniumcomplexes present in the nitric solution, and separating said organicphase containing the extracted ruthenium from said nitric solution.
 2. Aprocess according to claim 1, wherein the dialkyldithiophosphoric acidis di-(2-ethyl-hexyl)-dithiophosphoric acid.
 3. A process according toclaim 1, wherein the compound is sulphamic acid.
 4. A process accordingto claim 3, wherein the sulphamic acid concentration of the aqueousphase is between 0.05 and 1 M/l.
 5. A process according to claim 1,wherein the compound is hydrazine.
 6. A process according to claim 1,wherein extraction is carried out at a temperature between 20° and 80°C.
 7. A process according to claim 1, wherein the organic phase alsocontains a quaternary ammonium salt.
 8. A process according to claim 7,wherein the quaternary ammonium salt is tricapryl-methyl-ammoniumchloride.
 9. Application of the process according to claim 1, to therecovery of the ruthenium contained in a nitric solution obtained bydissolving irradiated fuel elements.